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LTEE populations on June 25, 2008. Richard Lenski that has been tracking genetic changes in 12 initially identical populations of asexual Escherichia coli bacteria since 24 February 1988. Over the course of the experiment, Lenski and his colleagues have reported a wide array of phenotypic and genotypic changes in the evolving populations. These have included changes that have occurred in all 12 populations and others that have only appeared in one or a few populations. For example, all 12 populations showed a similar pattern of rapid improvement in fitness that decelerated over time, faster growth rates, and increased cell size. The long-term evolution experiment was designed as an open-ended means of empirical examination of central features of evolution. To examine the dynamics of evolution, including the rate of evolutionary change.

To examine the repeatability of evolution. To better understand the relationship between change on the phenotypic and genotypic levels. As the experiment has continued, its scope has grown as new questions in evolutionary biology have arisen that it can be used to address, as the populations’ evolution has presented new phenomena to study, and as technology and methodological techniques have advanced. Moreover, due to the long use of E . Term paper lab chose to carry out the experiment with the bacteria grown in a glucose-limited minimal medium called DM25, which was initially developed by Bernard Davis for use in isolating auxotrophic mutants of E.

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Lenski’s laboratory at Michigan State University. The dilution means that each population experiences 6. 64 generations, or doublings, each day. Lenski chose to use in the long-term evolution experiment was derived from «strain Bc251», as described in a 1966 paper by Seymour Lederberg, via Bruce Levin, who had used it in a bacterial ecology experiment in 1972. Much analysis of the experiment has dealt with how the fitness of the populations relative to their ancestral strain has changed.

All populations showed a pattern of rapid increase in relative fitness during early generations, with this increase decelerating over time. Of the 12 populations, six have so far been reported to have developed defects in their ability to repair DNA, greatly increasing the rate of mutation in those strains. All twelve of the experimental populations show an increase in cell size concurrent with a decline in maximum population density, and in many of the populations, a more rounded cell shape. Over the course of the experiment, the populations have evolved to specialize on the glucose resource on which they grow.

This was first described in 2000, when Cooper and Lenski demonstrated that all populations had experienced decay of unused metabolic functions after 20,000 generations, restricting the range of substances on which the bacteria could grow. Two distinct variants, S and L, were identified in the population designated Ara-2 at 18,000 generations based on their formation of small and large colonies, respectively. Clones of the S and L types could co-exist stably in co-culture with each other, indicating they occupied distinct niches in the population. In 2008, Lenski’s team, led by Zachary D. Blount, reported that the ability to grow aerobically on citrate had evolved in one population. Around generation 33,127, a dramatic increase in turbidity was observed in the population designated Ara-3.

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